Turritopsis dohrnii is a very, very special jellyfish. It is tiny, about seven millimeters in length, yet it is immortal. This hydrozoan is capable of reversing its life cycle , returning to the form of a polyp, which is its state of sexual immaturity, even though it has already reached maturity. If most of the living beings are heading towards aging after passing the reproductive stage, Turritopsis dohrnii is reborn. And it does it over and over again, unless another animal eats it or a swimmer catches it.
The “immortal jellyfish” acts in the following way. After the male and female gametes unite, a larva emerges, which will settle on the seabed as a polyp (immature form). The polyps are then released as ephyrae, which are young jellyfish about to become sexually mature. These jellyfish will reproduce sexually and the cycle will begin again. If the jellyfish perceive some environmental threat, that is, if they become stressed, after reproducing they return to the polyp phase. They can also regress by genetic will. The process can be repeated indefinitely , hence they are considered biologically immortal.
Unsurprisingly, the jellyfish’s ability to evade natural death has made it a subject of study, not only biologically but also for those investigating the aging process in humans. Its ability to rejuvenate is precisely what attracted the interest of the professor of Biochemistry, an eminence in the study of cancer and National Research Award, Carlos López-Otín, and his team from the University Institute of Oncology of the University of Oviedo to study this tiny jellyfish
López Otín and his team have sequenced the genome of this mysterious creature and have compared it with that of another jellyfish that is a close relative and is deadly, T. rubra . The results of the study have been published in PNAS .
To carry out the study, they placed the specimens of Turritopsis dohrnii in fish tanks in the laboratory. The animals were captured not without difficulty and specimens from Italy, the Balearic Islands and even Japan were brought to Asturias. It was not easy to locate them due to their tiny size and because the polyps of T. dohrnii resemble those of other jellyfish. The jellyfish were fed on crustaceans and had their genome sequenced at different times in their lives as they grew. The scientists detected several changes in the genome , all related to DNA replication and repair . They observed changes in those genes involved in the oxidative stress that occurs as we age, as well as in the length of telomeres, also associated with aging. The investigation would come to demonstrate, therefore, that there is not a single key to rejuvenation and immortality, but that there would be several mechanisms that would enter the scene.
When the jellyfish reverses its life cycle, it gets smaller and its tissues change. The researchers also observed that certain genes stopped expressing (working) while others were activated. The GLI3 gene , for example, is involved in the differentiation process of pluripotent stem cells, that is, those that have the potential to adopt various forms in our body, in other types of cells. Well, T. dohrnii has twice as many copies of this gene compared to its relative T. rubra , and all copies remain active during the reversion process. This process is known as cell transdifferentiation : a cell transforms into a different type of cell, without needing to be a stem cell, giving rise to other cells outside the already established differentiation pathway. In humans this has only been described in Barrett syndrome.
Before focusing on T. dohrnii , López-Otín’s team deciphered the genomes of other especially long-lived creatures such as bowhead whales, which can reach 200 years, and the giant tortoises of the Galápagos. Human beings are very different from jellyfish, but thanks to this study, the researchers have been able to identify relevant genes that are involved in the rejuvenation process and could subsequently investigate how they work in humans.
Reference: Pascual-Torner, M., Carrero, D. et. to the. Comparative genomics of mortal and immortal cnidarians unveils novel keys behind rejuvenation. PNAS. 2022. DOI: https://doi.org/10.1073/pnas.2118763119
